The invention relates to a monochrome cathode ray display tube of the type having a display screen and an electron gum assembly for producing an electron beam and a deflection unit mounted on said display tube such that their longitudinal axes substantially coincide. The deflection unit comprising a line deflection coil system which when energised deflects the electron beam in a first direction, a field deflection coil system which when energised deflects the electron beam in a direction transverse to said first direction, an annular core member of soft magnetic material surrounding at least the line deflection coil system. The deflection unit has a first end facing said display screen and a second end adjacent said electron gun assembly. The deflection unit, when energized, produces dipole magnetic deflection fields resulting from said line and field deflection coils of substantially the same shape.
The deflection unit for deflecting the electron beam is used to deflect the electron beam from its normal undeflected straight path in one or in the other direction so that the beam impinges on selected points of the display screen so as to provide visual indications thereon. By varying the deflection magnetic fields in a suitable manner, the electron beam can be moved over the vertical display screen either upward or downward and to the left or to the right. By simultaneously modulating the intensity of the beam a visual presentation of information or a picture can be formed on the display screen. The deflection unit, which is coaxially arranged around the neck portion of the cathode-ray tube comprises two deflection coil systems so as to be able to deflect the electron beam in two transverse directions. Each system comprises two coils which are positioned on oppositely located sides of the tube neck with the systems being arranged around the tube neck 90.degree. relative to each other. Upon energization, the two deflection coil systems produce orthogonal deflection fields. The fields are essentially perpendicular to the path of the undeflected electron beam. A core of magnetisable material, which for deflection coil systems of the saddle type is situated closely around these systems, serves to concentrate the deflection magnetic fields and to increase the flux density within the tube neck.
Most prior art combinations of cathode-ray tube-deflection yoke have been manufactured for consumer television apparatus typically having 625 lines per frame (picture). Due to their restricted resolving power such combinations are not suitable for the display of texts or graphic representations. Thus there is a demand for monitors having a high resolving power which are designed so as to be able to display texts and graphic data much more clearly than the apparatus for domestic use.
In such monochrome cathode-ray tubes of high resolving power (hereinafter termed monochrome DGD (Data Graphic Display), a larger number of lines per frame is employed than is usual and also at a higher frequency.
In such tubes, certain requirements must be met. The spot must be sufficiently small in the centre of the screen and any distortion must remain particularly small upon deflection over the screen.
The first requirement can be fulfilled by using rotationally symmetrical converged electron beams having a comparatively large angular aperture (on the basis of the law of Helmholz-Lagrange). (Since the electron beam upon deflection becomes overfocused as a result of the curvature of the field, it is usual to use dynamic focusing to correct for this). However, when using a beam having a large angular aperture in general there is another spot growth mechanism which deteriorates the spot upon deflection of the beam, so that it is difficult to simultaneously satisfy the second requirement. A further requirement in monochrome DGD's is for very small North-South and East-West raster distortion.
In the conventional DGD deflection units which generate substantially homogeneous deflection magnetic fields, the spot quality can be maintained within acceptable limits but this is at the expense of the North-South and East-West raster distortion. Although the raster distortion can be compensated for electronically in the deflection circuit while maintaining the spot quality, this solution is economically unattractive. There is also a solution which needs no electronic correction in the deflection circuit. However, this involves the use of strong static magnets on the screen side of the deflection unit for the correction of the raster distortion, which has the disadvantage that upon deflection of the beam the magnets deteriorate the spot quality. If one is not satisfied with the spot quality which is achieved with this method, this can be improved by using so-called 4-pole corrections on the gun side of the deflection unit. These 4-pole corrections have even been considered to be indispensible when an extremely high resolution is desired (this requires the use of an electron beam having a very large angular aperture). For economic reasons such dynamically driven 4-pole corrections are to be avoided.